Classification, or taxonomy, is a system of categorizing living things. There are seven divisions in the system: (1) Kingdom; (2) Phylum or Division; (3) Class; (4) Order; (5) Family; (6) Genus; (7) Species.
Kingdom is the broadest division. While scientists currently disagree as to how many kingdoms there are, most support a five-kingdom (Animalia, Plantae, Protista, Monera, and Fungi) system. The lowest division is species, which consists of organisms that are capable of interbreeding to produce fertile offspring. Species are identified by two names (binomial nomenclature). The first name is the genus, the second is the species.
For example, a lion is Panthera leo, a tiger is Panthera tigris. The first word is always capitalized, the second is not, and both should be italicized. Humans, of course, are Homo sapiens. The full classification for a lion would be: Kingdom, Animalia (animals); Phylum, Chordata (vertebrate animals); Class, Mammalia (mammals); Order, Carnivora (meat eaters); Family, Felidae (all cats); Genus, Panthera (great cats); Species, leo (lions).
See also: Systematics, Taxonomy, and Classification.